1. Field of the invention
The present invention relates to a carbureter for an internal combustion engine, and more particularly it relates to a carbureter structure whose body is made of a soft material, and also to a structure for the attachment of the carbureter to the internal combustion engine.
2. Description of the prior art
As a system allowing an easy attachment of a carbureter to an internal combustion engine, there has been proposed in, for example, Japanese Utility Model Publication No. Sho 54-19454 an arrangement that the body of the carbureter is disposed between its upper-stream member and a lower-stream manifold pipe, and the body of carbureter is fixed by bolts as these bolts are passed through the three members. However, in case this known system is applied directly to a carbureter whose body is made of a soft material such as a synthetic resin, there arise such problems that leak occurs at the sealing sites due to stress relief (meaning the phenomenon that stress becomes weakened with time when strain is constant), or that the fastening bolts and nuts become loose. In order to solve these problems, there has been proposed a system to use reinforcing members having a length corresponding to the full length of the body of carbureter. It is, however, difficult from the practical point of view to make the total length of the carbureter body agree to the full length of the reinforcing member. In case the length of the reinforcing member is smaller than the length of the carbureter body, the reinforcing effect of these members will reduce substantially. Also, as disclosed in, for example, U.S. Pat. No. 4,414,163, such reinforcing members as mentioned above are passed through the bores which are provided in the flange portions (each sharing a same plane with its mating attachment end face) which, in turn, extend laterally from the end faces of the carbureter body intended for its attachment. According to this known carbureter-attaching method, the reinforcing members could easily come off, so that there is needed such an art of operation as forced inserting or caulking, with the result that there arises the problem that the attaching operation of a carbureter becomes considerably complicated and troublesome. Furthermore, since the opposite two attachment faces of the carbureter body and the entire faces of the flange portions are in direct contact with the attachment faces of both the manifold pipe and the air cleaner, there also has been the problem that it is not possible to prevent the occurrence of adverse effects of the heat of the engine upon the carbureter body made of a soft material. Also, in case the body of carbureter is made of a soft material such as a synthetic resin, there has arisen the problem that the fastening force of the inserted bolts tends to cause strains to the shaft-bearing bores which have been formed in the carbureter body to receive and support the choke valve shaft and the throttle valve shaft, and to develop deformations of the originally circular cross-section of the bores, with the result that the choke valve and/or the throttle valve develop impaired functions. Therefore, in the past, this problem has been solved by forming the shaft-bearing bores to have a somewhat larger diameter. This, however, has resulted in the development of a play on the part of the shafts which are passed therethrough, so that the choke valve and/or the throttle valve would rotate easily due to the vibrations caused, thus developing mal-functions.